Removed some IE6 specific code - saves 25 gz bytes

Some IE6 specific code was adding complexity to the `ajax.js` file. By refactoring it, for the `master` branch, we can improve readability and save 25 gz bytes.

```
Running "compare_size:files" (compare_size) task
   raw     gz Sizes
246147  72939 dist/jquery.js
 84233  29457 dist/jquery.min.js

   raw     gz Compared to master @ d837f119c3729565103005d5d7fa89e1dd8110cb
  -348   -113 dist/jquery.js
   -66    -25 dist/jquery.min.js
```
For reference: https://github.com/jquery/jquery/pull/1577/files#r12511062